The present invention relates to the balancing of high speed rotational members and more particularly high speed rotational turbine wheels used in association with turbomachinery.
Inherent in all high speed rotating machinery is the critical problem of balance. This is especially true in the case of turbomachinery where in certain applications the speed can be as high as 100,000 RPM and in cases where there is a need to retain close tolerance between the rotating and the stationary parts. Imbalance of high speed rotational members can lead to early bearing failure as well as what is commonly referred to as "noise". Noise is generally measured in terms of g-levels created by the imbalance of the rotating member. It is well known that imbalance in a rotating member is created by the unsymmetrical distribution of weight about the axis of rotation. While an imbalance in any one piece can be reduced to tolerable levels by the trial and error method of balancing through the removal of material or the addition of material to the rotating piece, this method is time-consuming and labor intensive and therefore uneconomical. Since a turbine wheel for use in a turbocharger is composed of generally three distinct pieces; an integral turbine wheel and shaft, compressor impeller, and a nut which secures the compressor impeller to the shaft at the end opposite the turbine wheel, each piece must be individually balanced. Furthermore, assembly of the three balanced pieces does not assure that the assembled article will itself be balanced.
Another method of reducing imbalance which has gained popularity with turbocharger manufacturers consists of the steps of balancing the assembled rotor-shaft assembly outside of the turbocharger housing and marking each piece of the assembly with a scribe line before dissasembly. Thereafter, the pieces are reassembled within the turbocharger housing using the scribe line on each piece to ensure that the pieces have the same angular relationship to each other that they had when originally balanced. This procedure does not guarantee a balanced reassembled rotor-shaft assembly but does improve the probability thereof.
However, assembly of any two or more balanced pieces can result in an imbalanced final product; this imbalance is known as "created imbalance" since the imbalance arises during the assembly of the balanced pieces. "Created imbalance" is caused by the mounting faces of the assembly being out of square (not perpendicular) with respect to the axis of rotation of the shaft. Hence, tightening of the nut causes bending of the shaft member, thereby destroying any balance the assembly may have had when originally balanced. To date there is no known method or means of reducing or preventing what has been described as "created imbalance" other than the two unsatisfactory methods outlined above.
Broadly speaking, the present invention discloses an apparatus and method for eliminating or reducing an imbalance created during assembly of balanced parts thereby eliminating the need for rebalancing and ensuring that the rotor-shaft assembly is balanced in its assembled state.
According to the present invention there is provided a turbocharger turbine wheel assembly comprising a turbine wheel having an integral shaft coaxial therewith, a compressor impeller and a nut. The compressor impeller includes a hub portion, a plurality of radially outward extending blades from the hub, an annular slot formed about the hub and a coaxial bore through the hub. Each piece is individually balanced before assembly. The compressor impeller is coaxially mounted to the shaft and retained thereon by the use of the nut which is threaded onto the shaft. During tightening of the nut any misalignment of the nut or compressor impeller which could cause bending of the shaft and therefore create imbalance is accommodated by compression of the annular slot in the compressor impeller nose.
It is an object of the present invention to provide an apparatus and means for reducing or eliminating any imbalance which could be created during assembly of the balanced components.
It is another object of the present invention to provide a method of eliminating the need to rebalance a turbocharger compressor impeller- turbine wheel assembly made of individually balanced components.
It is a further object of the invention to prevent the creation of any bending moment about the shaft during assembly.